Mice exposed to the high-energy particles found in space developed a harmful stress response.

Their radiated cells generated large numbers of free radicals - the destructive molecules that can damage DNA, leading to cancer.

The effect was most pronounced in the gut, where the response continued for up to two months. The mice also aged rapidly.

This kind of damage was likely to increase the risk of colon cancer, the scientists said yesterday during a presentation at the annual meeting of the American Association for Cancer Research in San Diego, California.

Dr Kamal Datta, who led the study, said: "With plans for a mission to Mars, we need to understand more about the nature of radiation in space."

"There is currently no conclusive information for estimating the risk that astronauts may experience."

The kind of radiation found in space is known as high linear energy transfer (LET) radiation.

It consists mainly of high energy sub-atomic particles and causes concentrated damage to living tissue.

Low LET radiation, such as gamma and X-rays which are used for medical imaging and treating cancer, tend to cause more diffuse damage.

Dr Datta's team found that low LET radiation did not generate free radicals in the same way as high LET radiation.

Solar flares pour out large quantities of high LET radiation which could present a real hazard to space farers.

People on Earth are shielded by the planet's atmosphere, but in space there is nothing to stop astronauts being blasted by the particles.